Explain the effect of when one impulse line is required for

Explain the effect of when one impulse line is required for measurement versus when two impulse lines are required?

Solution

Impulse lines are used for tapping fluid from a flow line, in order to feed it to a measurement device. Examples include the legs of a venturi meter, orifice meter etc.

In common applications, a double impulse line is usually used when the difference of head between two points in a flow is to be measured. On the contrary, single impulse line is often used when the absolute value of head is to be measured at some point in the flow.

Single impulse lines are usually open to the atmosphere, and hence the measurement obtained is usually gage value. For example, a Bourdon pressure gauge measures the difference between the pressure of gas and the atmospheric pressure. Whereas, the measurements obtained from a double impulse line is the change in the required measurand between the points of measurement. For example, the difference in static pressure head.